This invention relates to semiconductor packaging and, particularly, to flip chip packaging such as QFN (“Quad Flat No-lead”) lead frame packaging and, in some aspects, to flip chip BGA (“Ball Grid Array”) packaging.
A conventional QFN lead frame package includes a lead frame consisting of a centrally situated die paddle and peripheral leads, and a die attached to, and electrically connected with, the lead frame; and a molding or encapsulation that seals all of the parts of the die-and-lead frame assembly, and leaves land portions of the leads exposed for second-level interconnection of the package to circuitry in the environment of use, such as circuitry on a printed circuit board.
The die in a lead frame package may be a so-called flip chip die, which is situated in the package with the active side toward the lead frame, and which is attached and electrically connected to the leads by flip chip interconnections between peripheral pads on the die and bond sites on the leads. In some flip chip lead frame packages, there is no direct mechanical or electrical connection between the die and the die paddle, and electrical connections between the die and the lead frame are made only by way of connection of peripheral die pads with the leads. In many die, interconnect pads are situated in a central portion of the die in addition to the peripheral die pads; and particularly die pads for power and ground interconnection may be situated in a center portion of the die. In flip chip lead frame packages having such die, one or more electrical connections are made between interconnect pads situated in a central portion of the die and the die paddle.
Die having interconnect pads situated in a central portion of the die in addition to the peripheral die pads can alternatively be mounted on a substrate, such as a laminate substrate, comprising at least one dielectric layer and at least one patterned metal layer, as for example in a flip chip BGA package.
In a flip chip package, whether or not the die is electrically connected with the die paddle or the substrate, the active side is separated from the die paddle or the substrate. The separation must be sufficiently great to permit an inflow of the encapsulation material or molding compound between the die and the die paddle or substrate during the encapsulation or molding step, to reduce or eliminate formation of voids.
Moreover, where the flip chip interconnections are formed by remelt of solder, as the solder melts, the interconnect collapses and the die, not otherwise supported, moves toward the surface of the substrate or leadframe. As the interconnect collapses the solder can flow over any solder-wettable feature that it contacts and, accordingly, the molten solder can spread over a wide area of the lead or pad or leadframe. This can compromise the reliability of the interconnection.